In my opinion the strongest zipper is ykk nylon coil, for your application I would use a #5, the trick to any zipper is to keep your stitches close, what I mean don't stitch 3 stiches per in, go with 5 to 6 per inch an back off on the tention,if using steel tooth zipper make sure the zipper teeth are facing the same direction when putting on the zipper pull ( the teeth should be facing you as you put on the zipper pull. with nylon coil it don't matter